As is well known to those skilled in the art, ground anchors are widely used as a sheet for preventing the collapse of non-excavated ground around a construction field in underground excavation for subsurface structure works to construct a building or engineering structures, and as a safety measure for preventing a landslide of a cross section of poor ground. These ground anchors are of various types, i.e., a compression type, a tension type, and a pressure type. The compression type ground anchor is usually used but cannot remove a tension member. The tension type ground anchor is restricted to point pressure, thereby having difficulty in removing a tension member after works. Considering anchoring force, in the tension type ground anchor, the anchoring force is reduced by tension cracking of a grouting member. The pressure type ground anchor is applied only to rock bed capable of point bearing.
The ground anchor inserts an internal anchorage into a perforated anchor hole in rock bed by using a tension member (PC strand wire) with excellent tension strength, and maintains tensile force by imposing tensile load on a free long side. Therefore, if the tension member remains on the underground after completing the construction work of the subsurface structure, this tension member may be an obstacle to other construction works of the subsurface structure on ground adjacent to this field. In a downtown area with many buildings, a tension member removing method has been used more and more. An anchorage used in this case is an internal anchorage for removing the tension member.
Korean Patent Publication No. 96-4273 discloses an internal anchorage for removing a tension member of a ground anchor. This internal anchorage includes a body provided a solar wedge seat and a planet wedge seat around the solar wedge seat, the solar wedge seat dividing a circumference into two or three equal parts of the planet wedge seat, a solar wedge and a planet wedge seated on a corresponding seat, spacers in the same number as the planet wedge, an upper cover for preventing the upper separation of the solar wedge and the planet wedge, and a cap coating on the body.
However, in addition to the tension member having tension force engaged with the planet wedge, the above-described conventional internal anchorage requires a retrieval tension member for removing this tension member, i.e., the tension member engaged with the solar wedge, and another wedge for engaging this tension member. Therefore, the above-described conventional internal anchorage includes many components, resulting in difficulty in manufacturing, and further additionally includes the retrieval tension member for removing the conventional tension member, thereby increasing production cost. Moreover, it is troublesome to operate the added tension member for removing the tension member.
Further, the spacer includes a taper on its inner surface. The taper of the spacer corresponds to the tapered retrieval tension member. The outer surface of the spacer must be a circular section corresponding to a hollow core of the body. That is, since the spacer is very complicated in shape and structure, it is difficult to manufacture and assemble the spacer. By forming a ring groove on the back surface of a hole of a central member perforated in an axial direction and fixing a C-type retaining ring into the groove, the central member cannot be removed from the back surface of the body during the period in which tension force does not work. Since this internal structure is also very complicated, it is difficult to manufacture.
Furthermore, when the engagement of the tension member becomes loose prior to removing the retrieval tension member, if an anchoring load is imposed on the tension member, the retrieval tension member may slip out of the solar wedge. In this case, the retrieval tension member, which should retrocede by the solar wedge, does not retrocede. Thereby, the planet wedges do not open and the tension member cannot be removed. That is, the tension member cannot be removed without a drawbench.
In order to overcome the drawbacks of the above-described internal anchorage for removing the tension member of the ground anchor, Korean Patent Laid-Open No. 2002-47445 is described hereinafter. An internal anchorage of this document has a structure such that a wedge support formed by covering an electric heater with a thermoplastic resin is inserted into a body provided with one wedge groove, or at least two wedge grooves, and a wedge box is mounted on the wedge support. A tip of the tension member is engaged between the body and the wedge seat of the wedge box, and is inserted into an underground anchor hole and anchored. Then, the tension member is drawn and its outer end is anchored to a furring strip of soil wall. If current flows from the exterior to the wire connected to the electric heater after the work, the electric heater emits heat and melts the wedge support made of resin. The molten wedge support slips out of a space perforated on the back surface of the body. At this time, the wedge remains in its original position but the wedge box is pulled into the outer end of the tension member by tension force of the tension member. As a result, the engagement of the wedge with the tip of the tension member is released, and the tension member may protrude as a result of the anchoring load imposed on it, and may be removed.
However, when this internal anchorage for removing the tension member is molten by the heating of the electric heater of the wedge support, a spark is generated by the contact of the exposed portion of the wire connected to the electric heater with the metal-made body. Therefore, it is very dangerous. Further, if a short circuit is generated during melting of the wedge support, the wedge support is no longer molten and the wedge box does not pull out. Thereby, the wedge engagement is not entirely released and the tension member is not retrieved. Furthermore, since the resin of the wedge support is a special resin, it has several drawbacks, such as a low molding tendency and high production cost.